A highly advantageous application of the invention lies in improving the reliability of subjective evaluation of the voice quality of telephone calls conveyed by mobile networks. This subjective evaluation often leads to the voice quality of telephone calls conveyed by mobile networks being judged to be inferior to the voice quality of telephone calls conveyed by fixed networks. However, the voice quality of telephone calls constitutes an element that is changing. Firstly, the voice quality of mobile calls is expected to improve in the near future to reach the voice quality presently available from fixed networks. Secondly, transporting voice by means of packet switching techniques, in particular by so-called Internet Protocol (“IP”) techniques, might lead to the voice quality of fixed networks becoming degraded. Such degradation could occur specifically while implementing large-scale deployment of that type of transfer mode. This changing nature in the voice quality of calls requires there to be methods for evaluating such voice quality. Evaluation must be reliable so that any means defined on the basis of such evaluation and, in particular, any devices for correcting voice signals, such as echo cancellers, can be as effective as possible.
Two types of methods for evaluating voice quality have been used. A first type relies on objective methods of measurement that use test signals which reproduce signals similar to human speech. Voice quality evaluation is performed by analyzing physical characteristics of signals as received after they have been transmitted through a telecommunications network. Analysis can be time-based, frequency based, or it can relate to the power of the received signals. A drawback of those methods is that they use artificial signals that simulate speech rather than real speech signals.
A second type of method relies on subjective methods of measurement. This type includes a first category which utilizes human participants, referred to herein as experimenters, who passively listen to received signals and evaluate the signal quality. A second category makes use of active experimenters. Active experimenters converse in genuine telephone calls and then give their opinions concerning how they perceived the voice quality of the signals during the conversation. Although subjective, these methods are particularly useful in the field of voice signals since the ear constitutes a good instrument for measuring such signals.
Nevertheless, in order to obtain an accurate and reliable evaluation of voice quality, subjective measurement methods require a large number of experimenters. When the number is small, the accuracy of evaluation is highly dependent on the performance quality of each experimenter, and also on the conditions relating to the test environment.